ON SUPERSATURATFD SOLUTIONS. I3I 



I propose to show that neither exposure to air nor contact with 

 ordinary substances causes crystallisation in the great majority of 

 cases ; that the theories of M. M. Violette and Gernez and of 

 Professor Tomlinson are only applicable to a very limited number 

 of cases j that the contact theory of Jeannel breaks down utterly j 

 and lastly to give abundant experimental proof that the absorption 

 theory explains a great many cases of crystallisation. 



(i.) Air and ordinary substances do not generally cause super- 

 saturated solutions to crystallise. There is a very large number of 

 supersaturated solutions of salts of various kinds on the table, all 

 uncovered, they will remain liquid until I purposely make them 

 crystallise. Drops of these can be put on glass plates and on 

 various substances without their crystallising. They can be rubbed 

 with the linger, or with oil, or treated in various ways, still remain- 

 ing liquid. This method of working with drops is quite new and 

 is extremely convenient. A number of drops are put on a plate, 

 and a great many experiments can be tried as to the effect of 

 different substances. The trouble of boiling is thus reduced to a 

 minimum. The way in which the statement given above has got 

 into our textbooks can be explained thus. When I first began 

 working with these solutions I adopted the plan recommended in 

 the books of covering the solutions, after boiling, with cotton 

 wool. Under these circumstances removal of the cotton wool 

 generally caused crystallisation. But I noticed that the cotton wool 

 was often sticking to the mouth of the flask, and it struck me that 

 very probably in removing the wool small portions were detached 

 and fell in and caused crystallisation. I then tried paper and 

 tinfoil, and found that these could be removed any number of 

 times without causing crystallisation. After boiling has ceased a 

 good deal of spurting goes on for some time, the ascending currents 

 of air are seen to contain many little bubbles or drops of the 

 solution, and I have little doubt that these crystallise on the under 

 side of the cotton wool and are shaken in when this is removed. 

 It is also possible that the fragments of cotton wool cause crystalli- 

 sation by absorption. 



